For long years, contact lenses were predominantly produced as a hard lens (RGP) having a high oxygen permeability, and a soft contact lens (SCL) consisting mainly of hydro-gel which forms gel by absorption of water. The former, based on high oxygen permeability, has an advantage that a risk of obstructing cornea physiology activity is low, whereas it involves a problem of an inferior feeling in use because of its hardness. On the other hand, the latter, consisting of hydro-gel, is excellent in a feeling of use, but it involves a problem of low oxygen permeability.
In order to solve these problems, silicone hydro-gel has been developed as a high oxygen permeability soft contact lens material. Silicone hydro-gel is formed by producing a copolymer of an oxygen-permeable silicone component, like RGP, and a hydrophilic component used for the hydro-gel. Particularly, it is well-known to use, as a main ingredient of the silicone hydro-gel, a copolymer of a hydrophobic and high-molecular weight dimethylsiloxane macro monomer and a hydrophilic component (Patent documents 1-4). The silicone hydro-gel having thus a combination of mutually contradictory components is a material attracting attention these days, because it provides a good feeling in use like a conventional SCL, while satisfying a high oxygen permeability comparable to that of a conventional RGP.
However, in the case where the silicone hydro-gel is formed as a copolymer, even when ingredient materials of mutually contradictory properties appear to be mixed uniformly in a step of mixing them, the resultant mixture can result in an opaque copolymer, thus causing devitrification, after a polymerization thereof, in some cases. This is a fatal defect for the contact lens which is an optical product. Thus, the use of a silicone copolymer has realized an oxygen permeability higher than the hydro-gel lens, whereas it becomes a polymer with a higher elasticity than the conventional hydro-gel. Therefore, when it is used as a contact lens, it provides an inferior feeling in daily use, compared with a conventional hydro-gel lens, and can even result in lesions of anterior epithelium of cornea, generically called SEALs (Superior Epithelial Arcuate Lesions) peculiarly caused by wearing silicone hydro-gel lenses.